Monday, 4 December 2006

The magician throws the ball twice into the air and catches it, then he throws it a third time and it vanishes! Of course, he’s really secreted the ball in the palm of his hand, so why do so many observers believe they’ve seen the ball vanish mid-flight?

Gustav Kuhn and Michael Land recorded the eye movements of 38 participants while they watched a video clip of this vanishing ball illusion.

On the final throw, the magician looks skyward as if the ball really has been thrown and this social cueing is crucial to the illusion. Half the participants were shown a version of the trick in which the magician looked at his hand on the final throw instead of looking skyward, and in this case only 32 per cent of the participants experienced the illusion, compared with 68 per cent of the participants who witnessed the trick performed properly.

Moreover, whereas the participants said they had kept their eyes on the ball, the eye movement analysis revealed that before each throw, the participants glanced at the magician’s face.

But there’s a way in which the participants’ eyes were not fooled by the illusion. Those participants who experienced the illusion said they had seen the ball leave the top of the screen, and they guessed the illusion was created by someone catching the ball off screen. However, their eyes were not tricked – the analysis showed they only looked at the top of the screen when the ball was really thrown.

“These results illustrate a remarkable dissociation between what participants claimed to have seen and the way in which their eyes behaved”, the researchers said.

Their perceptual experience was based on their expectation of what would happen to the ball (informed by the magician’s misleading skyward gaze), whereas their eye movements were controlled by actual visual input. The finding is consistent with the huge body of research showing that perception and action are based on separate visual systems in the brain.
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